Add getRevision to api

The revision can be useful for instance to check if a feature has been modified so I'd like to see it in api.

src/ol/observable.js

@@ -47,6 +47,7 @@ ol.Observable.prototype.dispatchChangeEvent = function() {
 
 /**
  * @return {number} Revision.
+ * @todo api
  */
 ol.Observable.prototype.getRevision = function() {
   return this.revision_;